1. Field of the Invention
The present invention relates to a method and apparatus for continuous casting solder onto discrete parts and to solder containing parts formed thereby and, in particular, to a continuous process for injecting molten solder into molds to form solder mounds and wherein the molds may be disposed against a substrate such as a multilayer ceramic package or other electronic substrate and the solder mounds then transferred from the mold to the substrate or the molds disposed against a backing plate with the molded solder mounds transferred from the mold to another electronic substrate.
2. Description of Related Art
Forming an electronic package whereby an electrical component such as a multilayer ceramic package is electrically connected to a substrate such as a card, a board, another connector and any other electronic part is well-known in the art. Surface mount technology (SMT) has gained acceptance as the preferred means of joining electronic devices together, particularly in high end computers. As circuit densities increase there is an increased demand for more I/O and the interconnections which are solder bonded must be smaller and more closely spaced. Additionally, the height of each interconnection must be distributed within a relatively small height tolerance to insure that all of the multilayer ceramic package locations will be electrically interconnected to a corresponding substrate contact location through the solder interconnections.
A myriad of solder structures have been proposed for surface mounting of one electronic structure to another. A typical surface mount process is performed by screening solder paste on conductive, generally metallic pads exposed on a surface of a first electronic structure or substrate. Other known surface mount technology uses solder balls rather than solder paste to provide the solder connecting structures. This technology is known as C-4 and by using solder balls, a more exact and somewhat greater quantity of solder can be applied then through screening. These type solder structures are used mainly to connect a chip to a multilayer ceramic package.
Another type of solder interconnection is a cast solder column which is mainly used to connect a multilayer ceramic package to a boar. Cast columns (CGA) are preferred over pin grid arrays (PGA) in that the nature of the joint (solder connection) offers a better electrical connection over the PGA, is less expensive to manufacture and compensates for thermal expansion differences between the ceramic package and the board. In general, pins are prefabricated and then attached to the bottom side of the package substrate. Sockets are attached to the board where the PGA is plugged in. The PGA process is costly compared to the IMS cast column process.
In the present wire column grid array process a eutectic paste is screened on the substrate pads, typically the bottom surface metallurgy (BSM) pads. The solder columns are loaded separately into a mold or formed in the mold and then aligned to the substrate BSM pads and reflowed. This two-step procedure leads to increased manufacturing cost and possible misalignment of columns to the BSM pads.
U.S. Pat. No. 5,244,143, to Ference et al. and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference, discloses an apparatus and method for injection molding solder mounds onto electronic devices. In general, the apparatus has a reservoir for molten solder which is disposed over a cavity in an injection plate. The injection plate is disposed over a mold having an array of cavities therein into which solder is injection molded. The mold is typically disposed over a workpiece such as a multilayer ceramic substrate and the cavities in the mold are aligned with electrical contact locations on the chip or substrate. The workpiece is heated and the molten solder is forced under gas pressure into the cavity in the injection plate disposed above the array of cavities in the mold and the molten solder is forced into the cavities. In one embodiment, the injection plate is advanced to slide over the stationary mold to wipe away the excess solder above the mold at a plurality of wiping apertures in the injection plate. The injection plate is then further advanced to a location having a nonsoldered wettable surface at which location the injection plate is removed. The mold is then removed to leave solder mounds disposed on the workpiece.
The above apparatus and method can also be used with a fixed injection molding apparatus and injection plate and moving the mold and workpiece. The apparatus and method of the patent can also be used in an automated manufacturing system for depositing an array of solder mounds onto substrates. For an automated system the patent describes using intermeshing conveyer belt loops to provide a completely automated apparatus and process for disposing solder mounds onto electronic devices. A more advanced embodiment is a reel to reel apparatus and process for Tape Automated Soldering.
In the injection molded solder process (IMS), molds such as column grid array (CGA) molds are passed under an injection head or plate which fills the mold with molten solder. The injection head must make direct contact with the surface of the mold while the solder is in a molten state to force solder into the mold cavities and to prevent solder which is in a reservoir on top of the injection head from spilling out. This is a problem when the injection head is lifted to injection mold additional parts moved under the head and it has been proposed that before the injection head is lifted that the molten solder in the reservoir be allowed to solidify. A second proposal utilizes a vacuum on the reservoir to lift and hold the molten solder prior to lifting the injection head. Both solutions are impractical in a manufacturing environment where continuous processing is highly desirable. In alternative processes where the injection head is not raised during the process to mold additional parts it has been found that conveyor belts and the like are not satisfactory because of the competing thermal (cooling and heating) forces inherent in the process which provide gaps between the molds being sequentially processed and/or other problems in feeding and injecting molten solder into the mold.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the invention to provide a continuous process for injecting molten solder into a plurality of molds sequentially using such processes as the injection molded solder (IMS) process.
A further object of the invention is to provide an apparatus which may be operated continuously to fill a plurality of molds sequentially with molten solder using processes such as the injection molded solder (IMS) process.
It is an additional object of the invention to provide substrates and in particular electronic substrates such as multilayer ceramic packages with injection molded solder mounds thereon prepared by the process and method of the invention.
Other objects and advantages of the present invention will be readily apparent from the following description.